High temperature electrostatic precipitator and method of operation



Sept. 18, 1962 w. s. BOWIE 9 HIGH TEMPERATURE ELECTROSTATIC PRECIPITATOR AND METHOD OF OPERATION Filed Sept. 16, 1960 2 Sheets-Sheet 1 INVENIOR WALTER S. BOWIE gum,

ATTO EY w. s. BOWIE 3,054,243 HIGH TEMPERATUR IC PRECIPITATOR 2 Sheets-$heet 2 Sept. 18, 1962 E ELECTROSTAT AND METHOD OF OPERATION Filed Sept. 16, 1960 if j M QMQN 'EIOVl'lOA NMOCIMVBHE INVENTOR WALTER S. BOWIE Patented Sept. 18, 1962 3,054,243 HIGH TEMPERATURE ELECTRQSTATIC PRECIPI- TATGR AND METHOD F OPERATIQN Walter S. Bowie, Morgantown, W. Va., assignor to the United States of America as represented by the Secretary of the Interior Filed Sept. 16, 1969, Ser. No. 56,607 7 Claims. (Cl. 55-11) (Granted under Title 35, US. Code (1952), see. 266) The invention herein described and claimed may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of royalties thereon or therefor.

This invention relates to the removal of particulate matter from gases by electrically imposing a negative or positive charge on the particulates and subjecting them to an opposing polarity sufficient to attract and hold them, thereby removing them from the gas stream. The invention is an improvement on the present commercially available electrostatic precipitators, and is applicable to both plate and pipe type equipment. More particularly, the invention involves means to permit a more emcient removal of suspended particles in gases at elevated temperatures.

Commercial applications of electrostatic precipitators are limited to approximately 1000 F. operating temperatures. Dust removal apparatus employing the electrical precipitation method are so designed that the voltage gradient across the electrodes is a maximum for peak removal efiiciency. At elevated temperatures, thermal ionization of the gas and increased thermionic emission from the cathode causes voltage breakdown, and, consequently, the maximum possible potential is reduced. Inasrnuch as the potential is decreased, the positive charge on the anode is reduced and the degree of solids removal from the gas is proportionally diminished. Since increased electron emission from the cathode is due to the higher kinetic energy in the molecules of the cathode at elevated temperatures, it was determined that retardation of thermionic emission by cooling the cathode with a gas stream would decrease the rate of electron emission. Achieved thereby was a decrease in the concentration of electrons, negative ions, or negatively charged particles in motion, eifecting a diminished current from cathode to anode and an increase in the maximum voltage which could be impressed between the electrodes. As a result of the increase the magnitude of the positive charge (attraction) on the anode was increased, and more dust was removed from the gas.

An object of the invention is an improved electrostatic precipitator wherein the dust removal efficiency is increased.

Another object of the invention is to provide in an electrostatic precipitator, means to lower the temperature on the surface of one of the electrodes by causing a cooling medium such as a coolant gas to fiow through the electrode whereby the rate of thermal electron emission is reduced and a higher voltage gradient may be placed across the electrostatic field between the electrodes.

A still further object of the invention is to provide in an electrostatic precipitator a cathode having a hollow center through which flows a cooling gas whereby the rate of thermal electron emission from the cathode is reduced causing fewer gas ions to be created and increasing the breakdown voltage across the field of the electrodes.

In order that the invention may be more clearly understood reference will now be made to the accompanying drawings which show by way of example a preferred embodiment thereof.

In the drawings:

FIG. 1 is a sectional elevational view of a pipe-type precipitator embodying the present invention, and a schematic showing of various input and output means therefor, and their connections to the precipitator; and

FIG. 2 shows graphical representations relating the temperatures of the gases being processed to corresponding breakdown voltages between the electrodes.

Referring to FIG. 1, the illustrated gas purification arrangement includes a laboratory pipe-type precipitator 1, comprising a vertically mounted, relatively long length of tubing 2, flanged at a short distance from both ends by collars 3 and 4, each formed by two contacting plates welded to the long tubing. Enciroling the outer wall of the tubing from the respective ends to the adjacent flanges are water jackets 5 and 6, each constructed from a short length of pipe larger in diameter than the long tube and placed concentric therewith. The short pipes are welded to their associated flanges and are covered by plates 7 and 8, whereby the chambers formed for the respective jackets by the surfaces of the tubes, pipes, and flanges, are enclosed. Plates '7 and 8 overlap the outer edges of the long tube to cover a portion of the open end therein, to form openings 9 and 10. Jackets 5 and 6 comprise water inlet and outlet pipes 11 and 12, and 13 and 14, respectively. Between flanges 3 and 4, and surrounding the long tubing 2, is a thick blanket of high temperature insulation 1.5. The lower end of the insulation is supported on flange 4, and the upper end thereof is secured in place on the long tube by a collar plate 16, close to the flange 3. Inserted through the insulation 15, and connected to communicate with the inside of the long tube 2, are conduit pipes 17 and 18. Pipe 17 located near the lower end of the tube, provides means for entering the gases laden with finely divided particles such as dust into the ionization section of the precipitator. Pipe 18 located near the upper end of tube 2, provides an outlet for the cleansed gases.

A hollow tubing 19, of relatively small diameter, is centrally located about the longitudinal axis of the long tube 2, and supported in the openings 9 and 10, of plates 7 and 8, by identical insulator assemblies 20 and 21, respectively. Considering assembly 20 as exemplary of the insulator assemblies, it comprises a hollow tubular support rod 22 having a hole drilled centrally through its length and countersunk at a head end 23 to provide a passage and an enlarged opening 24 into which is securely fitted one end of the tube 19. Extending from the head end 23 is a turned down section of the support having a thread 25 cut thereon from an outer edge to about half its length. On diametrically opposite sides of the remaining length of the turned down section are welded guide lugs 26. Electrical insulator elements 27 and 28 separated by insulator spacer 29, form a collar in openings 9, and extend above and below the opening. Concentrically located within the opening in the collar, and out of contact therewith, is the turned down section of the support, the latter being secured in place relative to the insulators, by means of an inner flange surface of its head end 23, contacting the outer surface of insulator 28, and a nut 29 screwed on the thread 24, tight against an outer surface of insulator 27.

A stand 30 having upright legs 31, welded to a base plate 32, and secured to the lower flange 4, supports the main precipitator structure at a height permitting connections to be made to inlet and outlet openings below the flange 4. An insulator coupling 33 connects to the upper support rod 22 a pipe 34 for supplying through the passage in rod 22, a coolant gas to the interior of tubing 19. By means of a similar insulator coupling 35, a pipe 36 is connected through the passage of lower support rod 22' to the hollow of tubing 19, for exhausting the coolant gas after its passage through the tubing. Conventional water supply and drain connections are made to the inlet and outlet pipes 11, 12, 13, and '14-, of the water jackets and 6.

Electrical connections to the precipitator include a conductive lead 37, joining the high potential side of a direct current source 38 to the electrically conducting support rod 22, and the tubing 19, and a second conductive lead 39 joining the ground side of source 38 to the long tube 2, through a conventional voltage control unit 40, and a lead 41. An electrical tubular heater arrangement 42 of conventional design, is spiraled around the long tube 2, and is connected through leads 43, 44 to a powerstat unit 45.

A consideration of the fundamental operative effects in an electrostatic precipitator may be begun by assuming a gas to be cleaned as admitted to long tube 2, from the inlet conduit pipe 17. With source 38 supplying a high potential to tube 19, a high voltage gradient exists between the tube 19, which may be considered the cathode, and the tube 2, which may be considered the anode. A high corona is set up around the cathode '19 which causes release of many electrons from the metal thereof. According to theory, the released electrons migrate toward the anode 2, and through collision with gas molecules in the long tube cause gas ions to be produced. The negative ions attach themselves to the particulate matter in the gas which thus become negatively charged. The finely divided particles are attracted to the anode, and are held there. As a result gases free of the particles pass through the precipitator and out conduit pipe 18.

Maximum voltage which can be applied across the electrodes 19 and 2 is a function of the degree of ionization of the gas, or gases occupying the space between them. At elevated temperatures the kinetic energy of the molecules in the metal of the cathode is increased, and additional electrons are released therefrom by reason of thermal excitation. Any increase in the rate of electron emission from this source provides more opportunity for collision between the electrons and the gas molecules, thus creating more gas ions. Therefore a means for decreasing the rate of emission will decrease the number of gas ions produced and as a consequence an increase in voltage can be applied before reaching the point of breakdown voltage. This increased voltage would result in a higher charge on the collecting electrode or anode in this case. Solid particles in the gases would therefore be subject to an increased rate of migration which results in more particles being attracted and held. Higher removal efficiencies are therefore made possible.

By means of the present invention the above indicated drawbacks found to be inherently part of the operation of the precipitator at elevated temperatures, is significantly diminished. Although the gases being cleansed in the precipitator may be as high as 1500 F. or higher, the cathode surface of the invention is maintained at a substantially lower temperature and the effect of the thermal excitation normally corresponding to the high temperature of the gas, is eliminated. Referring to FIG. 2, the two curves A and B, are based upon data determined in an exemplary precipitator arrangement, as shown in FIG. 1, wherein the long tube 2 was three inches in diameter, and approximately thirty inches in length. Curve A represents the relationship between the breakdown or maximum potential gradient measured between the electrodes, and the temperature of the gases to be cleaned in long tube 2, in the absence of coolants in the parts of the precipitator of FIG. 1, constructed to receive them. It will be noted that as the temperature of the gases in tube 2 increased from 700 to 1000 F., there is a sharp drop in the breakdown voltage, and a further sharp drop between the gas temperatures of 1200 to 1500 F. Curve B represents the same coordinate relationships when coolants were used in the parts of the precipitator constructed to receive them. Although curve B shows corresponding breakdown voltage drops between the temperature ranges previously cited, it also shows these drops to be significantly less than that made evident by curve A.

Cooling the cathode element or hollow tubing 19, is accomplished by passing one of the many known coolant gases or fluids from the pipe 34, through coupling 33, the passage in support rod 22, into the tubing, and out therefrom through corresponding outlet parts 22, 35, and 36. The coolant gas cycle can be open or closed. If it is closed, a heat exchanger must be installed in the system before the returned coolant gas is permitted to flow into an inlet of a gas pump 50. Thermocouples 46, 47, a rotometer 48, and a manometer 49, may be connected into the coolant gas line to measure temperature, flow rate, and pressure, respectively, so that the heat removed from the cathode element by the coolant gas may be calculated.

Water jackets 5 and 6 are provided in the vicinity of the insulator assemblies 20 and 21, to draw the heat therefrom and the ends of the tubing 19, to lower their operating temperatures, and to protect the insulators from destructive stresses due to the elevated temperatures.

To maintain the elevated temperatures for the cleansed gases flowing through long tube 2, the high temperature insulation 15 is provided to restrict heat losses, as well as to permit safe operation. Also provided to maintain the temperature for the gases in pipe 2, is the electrical heating arrangement 42, 43, 44 and 45.

The invention is clearly not limited to the vertical arrangement as shown in FIG. 1. Equipment can be mounted at any angle with the vertical and the cleansed gases do not necessarily need to pass up through the unit.

The efficient removal of dust in gases at elevated temperatures achieved as a result of the present invention permits the utilization of product and by-product gases containing dust in a gas turbine for generating additional energy. Other benefits flowing from this invention are decreased erosion in industrial equipment by removing the cause of erosion (dust) at temperatures at which heretobefore precipitators did not function satisfactorily, and the extension of the life of catalysts in certain industrial chemical processes by removing dust which otherwise would erode the catalyst pellets or clog the catalyst bed at high operating temperatures.

While a single tube precipitator has been illustrated, it will be appreciated that the same teaching may readily be applied to multiple tubes arranged as stages. Other modifications of the specific apparatus shown and described for use in practicing my invention, will also be readily apparent to those skilled in the art.

I claim:

1. In an electrostatic precipitator, a first hollow tubufi lar member having concentrically located therein a second hollow open ended tubular member and separate electrical connections of opposite polarities from a power source to the two members, insulator collars and mounting means therefor attached to the two members supporting the second member in its spacial relationship within the first member and electrically isolating the members one from the other, conduit means made integral with the first member providing a passage for the flow of gaseous material to be cleaned into and out of a space defined by the external surface of the second member, the internal surface of the first member and the insulator collars and the mounting means therefor, and further conduit means coupled to the open ends of the second member providing a passage for the flow of a coolant fluid into and out of the hollow portion of the second member.

2. In an electrostatic precipitator as defined in claim 1, separate annular chambers at both ends of the first member each surrounding a portion of the length of the outer surface thereof, pipe connections to said annular chambers providing passages for the flow of a coolant fluid to circulate into, out of and within the annular chambers.

3. In an electrostatic precipitator as defined in claim 2, a sleeve of high temperature insulation material surrounding the outer surface of the first member along a length thereon substantially equal to the entire length between the annular chambers, and a heating element uniformly spaced along the length of the outer surface of the first member Within the said sleeve of insulation material.

4. In an electrostatic precipitator suitable for purifying gaseous materials at elevated temperatures, an anode electrode comprising a long cylindrical casing, and a cathode electrode comprising a substantially equally long slender tubing, means having openings therein partially enclosing the ends of the cylindrical casing, electrical insulator means fixed in said openings, hollow rods secured in said insulators having means for supporting the tubing in a spacial relationship to the cylindrical casing and electrically isolated therefrom, conduit means made integral with the cylindrical casing providing a passage for the flow of gaseous materials to be cleaned into and out of a space defined by the outer surface of the tubing, the inner surface of the cylindrical casing, the insulator means and the structure connected directly thereto, and further conduit means coupled to the hollow rods and providing passages for the flow of a coolant fluid moving through the hollow parts of the rods and the tubing.

In an electrostatic precipitator as defined in claim 4, separate annular chambers at both ends of the cylindrical casing, each surrounding a portion of the outer surface of the casing adjacent the insulator means, pipe connections to said annular chambers providing passages for 6 the flow of a coolant fluid circulating into, out of and within the annular chambers.

6. In an electrostatic precipitator as defined in claim 5, a sleeve of high temperature insulation material encircling the outer surface of the cylindrical casing between the annular chambers, and a heating means attached to the outer surface of the cylindrical casing within the said sleeve of insulation material.

7. A method for efliciently operating an electrostatic precipitator at high temperatures, the steps of introducing in a flowing stream a particle laden gaseous material having an elevated temperature, between the electrically isolated surfaces of the anode and cathode elements of the precipitator, applying a source of high direct current voltage to the electrodes to produce a high voltage gradient between the electrodes, and applying a cooling medium to the structure of the cathode electrode to establish a substantial differential in temperature levels between the surface of the cathode electrode and the gaseous material flowing between the electrodes.

Stevens Apr. 14, 1925 Miller Oct. 25, 1932 

